A. Technical Field
The present invention relates to: a composite-oxide catalyst used for production of acrylic acid; and a process for production of acrylic acid using this catalyst.
B. Background Art
A large number of catalysts improved for efficiently producing acrylic acid by a catalytic gas phase oxidation reaction of acrolein have hitherto been proposed. For example, there have been disclosed the following catalysts: a catalyst including molybdenum, vanadium, and tungsten (e.g. refer to JP-B-012129/1969); a catalyst including molybdenum, vanadium, copper, tungsten, and chromium (e.g. refer to JP-B-011371/1974); a catalyst including molybdenum and vanadium (e.g. refer to JP-B-025914/1975); and a catalyst including molybdenum, vanadium, copper, and at least one of antimony and germanium (e.g. refer to JP-A-085091/1977).
However, these prior catalysts have a problem such that, when they are used for a long term, their catalytic performances gradually deteriorates due to sublimation of the molybdenum component. And this problem becomes more remarkable due to exposure of the catalysts to high temperature.
As one of means for solution of the above problem, there is disclosed a method for preventing the rise of the hot-spot temperature. For example, there are disclosed the following methods: a method in which a catalyst layer on the raw-gas-inlet side is diluted with an inert substance (e.g. refer to JP-B-030688/1978); and a method in which the supporting ratio of the catalytically active substance is increased in order from the raw-gas-inlet side toward the outlet side (e.g. refer to JP-A-010802/1995).
However, these methods are methods for preventing the hot spots from becoming too high temperature. Therefore, in these methods, the hot spots themselves remain formed as conventional. That is to say, the above methods can never be said to be thorough solutions of the catalytic-performance deterioration caused by the hot spots, and there is accordingly still room for improvement.
On the other hand, when acrylic acid is produced by a catalytic gas phase oxidation reaction of acrolein with a catalyst, usually not a few hot spots are formed in the catalyst, and it is therefore actually difficult to carry out the production with complete elimination of the hot-spot formation.